Abstract
The formulation of an adequate description of neutron populations in
the presence of boiling moderator or coolant is attempted. Starting from
a framework of comprehensive stochastic descriptions we concentrate on the
special requirements of the boiling system such as the use of transport
theory and the retention of heterogeneity in our model. Working equations
are derived which show how adequate corrections for boiling can be made to
the approximate models currently used.
The detailed distribution of bubbles in a boiling system should be
accounted for in accurate evaluations of criticality, especially where the
bubbles are large. Evaluations of leakage, resonance escape probability
and thermal absorption are modified by the detailed bubble distribution.
Explicit corrections are given for simple examples, while inclusion of the
effects into a practical criticality code, such as WIMS, is discussed.
The second moments of the neutron population in the presence of
boiling media are also described. The power spectral density arising from
the fission process is evaluated, showing how this spectrum is affected by
the boiling. Usually more important sources of neutron noise in a boiling
reactor are the fluctuations in cross-sections and collision probabilities
arising directly from random local void ages. This 'bubble noise' is evaluated with particular emphasis on the detection of onset of boiling in a
channel or change in boiling regime.
The use of Monte Carlo techniques in calculating neutron transport
in boiling media is discussed in some detail. The applicability of the
analyses of neutron means and variances to fast reactor situations is
considered.
The bulk of the work treats critical reactors. However, a treatment
of transients and feedback effects is also included. A new analysis is
suggested whereby means and variances throughout the transient can be evaluated without extreme difficulty. The concepts of stochastic stability are discussed.